Basically, here are the things you need to know:1. If you want to compile with flex 3 SDK, you will need a new license for flex 3. The license for flex 2 sdk would not work. 2. To apply the license from FB3, go to Help –> Manage flex license, then type in the license key. It should look like this: 1377-xxxx-xxxx-xxxx-xxxx-xxxx.
3. To apply the license in flex-config.xml:
<flex-config>
<licenses>
<license>
<product>flexbuilder3</product>
<serial-number>1377-xxxx-xxxx-xxxx-xxxx-xxxx</serial-number>
</license>
</licenses>

<compiler>
…
</compiler>
</flex-config>

4. To apply the license in license.properties file:
The path to the license.properties file for various OS are changes for flex 3, they are now at:
1).”../Documents and Settings/All Users/Application Data/Adobe/Flex” directory on Windows
2).”Library/Application Support/Adobe/Flex” directory on MAC
3).”$HOME/.adobe/Flex” on Linux